1. Field of the Invention
The present invention relates to a method and an apparatus for drying a coated film. More particularly, it relates to a method and an apparatus for drying a surface of a wide and long continuous coated film formed by applying a coating liquid containing an organic solvent over a continuous support in a preparation process of an optical compensation film, etc.
2. Description of the Related Art
In order to improve viewing angle characteristics in a liquid crystal display device, an arrangement of an optical compensation film as a phase difference plate between a pair of polarizing plates and a liquid crystal cell has been adopted. For preparing the continuous optical compensation film, steps of applying a coating liquid containing a resin for forming an orientation film over a surface of continuous transparent film and forming the orientation film by subjecting to a rubbing treatment are carried out. Subsequently, applying a coating liquid containing a discotic liquid crystalline compound on the orientation film to form a coated layer and drying the coated layer are disclosed (see Japanese Patent Application Laid-Open No. 9-73081).
The drying method for the coating liquid containing the discotic liquid crystalline compound disclosed in the Japanese Patent Application Laid-Open No. 9-73081 employs an initial drying under a room air conditioning condition after applying the coating liquid containing the discotic liquid crystalline compound on the orientation film and before drying by means of a regular drying apparatus. In the initial drying, organic solvents in the coating liquid are mainly vaporized before drying.
However, in the optical compensation film prepared by the preparation method described in the Japanese Patent Application Laid-Open No. 9-73081, there is a problem that two kinds of unevenness (mottles) A and B which are illustrated in FIG. 10 such as a broad unevenness A (shown as thin lines) and a sharp unevenness B (shown as bold lines) occur on the surface of the coated film 101 in the initial drying step thereby decreasing the yield of the product depending on the cases.
As a result of analyzing those two kinds of unevenness A and B, it was clarified that the broad unevenness A reveals when the thickness of a film 102 layer of the coated liquid containing a discotic liquid crystalline compound becomes thin as shown in FIG. 11. In FIG. 11, a numerical code 103 illustrates a continuous support and a numerical code 104 illustrates an orientation film layer. On the other hand, it was clarified that an orientation direction 106 in an orientation part 105 (dense color part) where the sharp unevenness B is occurring deviates from an orientation part 108 of other normal orientation direction 107 as shown in FIG. 12.
A countermeasure generally executed as effective measures against those unevenness A and B which occur in the initial drying is to increase a viscosity of the coating liquid by elevating a concentration of the coating liquid or by adding a thickener. The countermeasure is a method of preventing the occurrences of unevenness by suppressing a fluid flow of the surface of the coated film induced by the drying wind immediately after coating. Another method is a method of preventing the occurrences of unevenness by causing a leveling effect with the usage of an organic solvent having high boiling point despite a generation of the fluid flow of the surface of the coated film induced by the drying wind immediately after coating.
However, to increase the viscosity of the coating liquid by elevating the concentration of the coating liquid or by adding the thickener has shortcomings failing to achieve super thin layer precise coating for forming a coated film of super thin layer by high-speed coating. Further, because the thicker increases the viscosity of the coating liquid, the lower a threshold coating velocity (threshold of the coating velocity capable of stable coating) falls, a high-speed coating becomes impossible with the increase of viscosity and accordingly, there is also a shortcoming that a production efficiency turns worse furiously.
On the other hand, the usage of an organic solvent having high boiling point induces augmentation of drying time and augmentation of amount of residual solvent remaining in the coated film thereby requiring much drying time and accordingly, there is also a shortcoming that the production efficiency turns worse.
From such a background, inventors of the present invention already proposed a method and apparatus for drying coated film described in Japanese Patent Application Laid-open No. 2001-170547. The method and apparatus provides a technology in which disposing a drying zone immediately after the coating, together with surrounding a surface of the coated film to be dried of the traveling continuous support, and generating a one-way flow drying wind flowing from one edge side to another edge side in a width direction of the continuous support resultantly achieving uniformly drying the coated film without changing the property such as viscosity and so on or the kinds of the solvent. The method and apparatus for drying the coated film is estimated as capable of suppressing the above occurrences of unevenness.